The industrial preparation of urea is at present based virtually exclusively on the high-pressure synthesis from ammonia and carbon dioxide in a urea plant at about 150 bar and about 180° C.
The two starting materials for the urea synthesis are generally provided in an ammonia plant which is usually in the direct vicinity of the urea plant concerned. The carbon dioxide required for the urea synthesis is obtained in the synthesis gas production of the ammonia plant as constituent of the crude synthesis gas after reforming. Since carbon dioxide would act as catalyst poison in the ammonia synthesis, it has to be separated off from the synthesis gas. In the prior art, the use of regenerative gas scrubs, for which there is a relatively large number of selectively acting solvents available, is known for this purpose.
Since ammonia is generally present in liquid form at the battery limits of the ammonia plant, it can be brought to the pressure level of the urea plant only with a limited outlay in terms of energy and apparatus. However, the carbon dioxide is obtained in gaseous form in the ammonia plant. For this reason, a disproportionately large outlay in terms of energy and apparatus is required for increasing the pressure in order to bring the carbon dioxide to the pressure level of the urea plant.
The integration of ammonia and urea plants, in which the carbon dioxide is separated off using ammonia or ammonia/water mixtures, is known in the prior art. The carbon dioxide is predominantly bound chemically in the form of carbamate ions and carbonate ions in the solution and can then likewise be introduced into the urea synthesis with a comparatively low outlay by pumping.
In the past, various concepts for process-side integration of ammonia and urea plants have been proposed in order to reduce the total energy requirement for CO2 compression and also the overall outlay in terms of apparatus for the plant. A similar aspect of all these concepts is that the entire amount of CO2 is removed from the synthesis gas by means of ammonia intrinsic to the process and the mixture formed is passed without further work-up to the urea synthesis. This is referred to as a “fully integrated ammonia-urea complex”.
DE 1 668 547 discloses a process for preparing ammonia and urea which is characterized in that ammonia synthesis gas containing carbon dioxide, nitrogen and hydrogen is introduced into a first zone which is kept under such conditions that carbon dioxide is separated off from the synthesis gas in an ammonia-containing liquid and a condensate containing ammonium carbamate is obtained and in that the remaining ammonia synthesis gas is introduced into an ammonia synthesis zone and the condensate is introduced into a second zone which is maintained under conditions suitable for the preparation of urea from the condensate. This is a typical example of a fully integrated process, and no direct isolation of CO2 takes place here.
DE 26 13 102 C2 discloses a process for the simultaneous preparation of ammonia and urea, in which a gas mixture which is composed of carbon dioxide, nitrogen and hydrogen and has been obtained in the reforming of hydrocarbons and subsequent CO conversion is fed to an absorption of the carbon dioxide by means of ammonia solution which is obtained in an absorption of the ammonia from the ammonia synthesis by means of water and the ammonium carbamate solution formed in this way is introduced into the urea synthesis.
In both these processes, the ammonium carbamate solutions are passed directly to the synthesis of urea. However, detailed studies have shown that it is not possible to realize an energetically favorable overall solution in this way. The synthesis of urea from ammonia and carbon dioxide is exothermic overall. It consists of the relatively strong exothermic and comparatively fast reaction of the starting materials to form ammonium carbamate and the significantly slower and endothermic decomposition of the carbamate to form urea and water. Good energy efficiency of the overall process can be achieved only when the heat of reaction liberated in the carbamate formation reaction is utilized for the formation of urea.
In addition, it is a fact that the start-up of an integrated plant made up of ammonia and urea plants is complex and separate operation of ammonia or urea plant would not be feasible.
A large quantity of heat is liberated in carbamate formation in a CO2 scrub using ammonia or ammonia/water mixtures. Water could take up the heat much more effectively than ammonia, but is present only in small amounts, if at all. Cooling therefore has to be employed to hold back the ammonia. However, in the concepts proposed hitherto, this heat mostly has to be removed unutilized by means of cooling water. The quantity of heat required by the urea reactor then has to be additionally provided and the energy balance of the process becomes more unfavorable. Furthermore, a considerable additional quantity of water is introduced with the carbamate stream into the urea synthesis in the processes described in the prior art, as a result of which the equilibrium of the urea formation reaction is adversely affected. Without additional introduction of water, the urea yield is in the ideal case about 45%. Each additional introduced water molecule decreases the yield. Comprehensive information may be found in the literature, e.g. in S. Kawasumi, Equilibrium of the CO2—NH3—H2O-Urea System under High Temperature and Pressure. III. Effect of Water Added on Vapor-Liquid Equilibrium, Bull. Chem. Soc. Jap, Vol 26 (1953), No. 5, pp. 218-222.
DE 32 39 605 A1 discloses a process for the combined preparation of ammonia and urea, in which the ammonia synthesis gas consisting essentially of hydrogen, nitrogen and carbon dioxide is subjected to a pressure scrub at temperatures of not more than ambient temperature in order to remove acidic impurities, in particular carbon dioxide, using a physically acting solvent, whereupon the loaded solvent is partially depressurized to effect outgassing of inerts and subsequently regenerated at atmospheric pressure and recirculated to the pressure scrub and the carbon dioxide liberated in the regeneration is employed for the urea synthesis. However, this process makes only a small reduction in the work of compression possible. In addition, only part of the carbon dioxide is liberated from the solvent at a relatively high pressure in this process. A CO2 compressor is therefore also still necessary in this process.
Thus a need exists for improved processes and apparatuses for preparing urea in a fully integrated plant having a nominal output of, for example, 1000 metric tons per day.